Butterfly valves are used in many container outlets and in railway hopper car and tank car outlets.
However, to open most butterfly outlets, the operator must overcome the weight of the lading as the valve is rotated from a horizontal position, closing the outlet, to a vertical position allowing lading discharge.
In U.S. Pat. No. 2,878,829 (1959), a circular plate containing a pair of spaced openings is disclosed which in open position align with channels or passageways in a housing located on top of a railway tank car. One channel or passageway constitutes a siphon conduit outlet. The other passageway constitutes a tank car vent port. The circular plate is moved between open and closed positions by a pinion which engages rack teeth located on the external surface of the circular plate.
However, the flat surfaces of the housing and circular plate would not be adapted for use as a lading outlet valve in the bottom of the tank. Particulate and viscous ladings would tend to cause the circular plate to bind against the housing, making the outlet difficult or impossible to open.